Effects of Solcoseryl were studied in vivo on Ehrlich ascites mice and mammary carcinoma in C(3)H mice. The results were as follows: 1) Solcoseryl did not prolong the servival time of Ehrlich ascites mice. 2) Solcoseryl had a stimulating action on tumor growth of mammary carcinoma in C(3)H mice. 3) There was no difference of servival times between Ehrlich ascites tumor mice treated with combination use of Solcoseryl and Mitomycin C or Toyomycin and those with single use of Mitomycin C or Toyomycin.